UV reactive spray chamber for enhanced sample introduction efficiency

ABSTRACT

An analyte for atomic spectrometry detection is prepared by introducing an aerosol of the analyte into a chamber, and irradiating the aerosol with ultraviolet light in the presence of a low molecular weight organic acid or other suitable photoactivatable ligand donor species to create vapor containing the analyte. The vapor containing the analyte is extracted from the chamber and used for atomic spectrometry detection.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 USC 119(e) of U.S.provisional application Ser. No. 60/635,447, filed Dec. 14, 2004.

FIELD OF INVENTION

This application relates to an apparatus and a method for generating agaseous form of an element from a liquid sample containing the element.

BACKGROUND OF THE INVENTION

Atomic spectrometry detection frequently requires the ready availabilityof a liquid sample. Conventional sample introduction techniques foratomic spectrometry detection rely predominantly on pneumaticnebulization of liquids.

There are several techniques in current use for vapor generation, butthis is classically accomplished using chemical derivatization reactionswhich are conducted in separate modules and frequently independent ofthe sample nebulization process. The most popular of these techniques isthe so called hydride generation approach, which relies on the reductivehydridization of a small number of elements by the action of an aqueoussolution of sodium tetrahydroborate. This approach, as well as othersrelating to halide generation and aqueous alkylation reactions forgeneration of volatile slightly water soluble forms of metals isdiscussed in R. E. Sturgeon and Z. Mester, Analytical Applications ofVolatile Metal Derivatives, Appl. Spectrosc. 56 202A-213A (2002).

These metal vapour generation protocols are limited in scope to ahandful of elements and are themselves difficult to implement,frequently requiring separate gas-liquid separators and excluding allother elements not amenable to the derivatization reaction.

Enhancement of sample introduction efficiency is currently being pursuedby many practitioners of atomic spectrometry. Current activity includesthe design of improved nebulizers and spray chambers, frequentlyoperating at low sample uptake and ultimately relying on theirintegration or complete elimination of the latter so as to achieve 100%efficiency or utilizing chemical vapor generation (CVG) to convert theanalytes of interest to volatile species, thereby achieving similarresults. CVG is undergoing a resurgence of interest in the past decadefollowing the report of a volatile species of copper generated duringmerging of an acidified solution of the analyte with that of sodiumtetrahydroborate reductant. Subsequently, a number of transition andnoble metals have been detected based on similar reactions, buttypically under conditions facilitating rapid separation of therelatively unstable product species from the liquid phase. Thisrequirement is most easily met when the sample and reductant solutionsare merged at the end of a concentric or cross-flow nebulizer, theresultant aerosol providing a unique atmosphere for rapid release of thevolatile product from a large surface-to-volume phase into an inerttransport gas.

A simplified and potentially “cleaner” arrangement for vapor generationcan be realized with the use of ultraviolet irradiation. See, forexample, X. Guo, R. E. Sturgeon, Z. Mester and G. J. Gardner, UV VaporGeneration for Determination of Se by Heated Quartz Tube AAS, Anal.Chem. 75 2092-2099 (2003). Although UV has been widely deployed toassist with oxidative sample preparation, its application as a tool foralkylation of a number of metals has only recently emerged. Radicalinduced reactions in irradiated solutions of low molecular weightorganic acids provide small ligands capable of reducing, hydrogenatingand/or alkylating a number of elements to yield volatile products. X. M.Guo, R. E. Sturgeon, Z. Mester and G. J. Gardner, Anal. Chem., 2004, 76,2401-2405.

To date, the process of photoalkylation for analytical purposes(enhanced detection capability for metals, semi-metals or non-metals)has been achieved using either one of two approaches: irradiation ofsample in a batch reactor containing the analyte element of interest andthe LMW acid which is connected to analytical instrumentation used forelement detection via a gas transport line; or by irradiation of acontinuous flowing stream of sample containing the analyte element ofinterest and the LMW acid which is directed to a gas-liquid separatorfor phase separation and transport of a carrier gas containing thegenerated analyte to the detection system. These techniques are not,however, suitable for efficient sample preparation for atomicspectrometry equipment.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodpreparing an analyte for atomic spectrometry detection comprisingintroducing an aerosol of the analyte into a chamber; irradiating theaerosol with ultraviolet light in the presence of a low molecular weightorganic acid or other suitable photoactivatable ligand donor species tocreate a reduced, hydrogenated and/or alkylated and/or elemental vaporcontaining the analyte; and extracting the vapor from the chamber foruse in atomic spectrometry.

The analyte can typically be metallic or metalloid elements, and any nonmetallic elements from main groups V, VI and VII of the periodic tablethat form volatile adducts, such as transition metals, heavy metals,semi-metals, halides and precious metals.

The low molecular weight organic acid or other alkyl donor speciesshould provide a concentration of 1000 times the molar level of theanalyte, preferably from 0.001 to 10 M, and more preferably from 0.01 to10M.

Ultraviolet light is suitable for the method. If the wavelength is toohigh, above about 400 nm, no reaction is observed. If the wavelength isto low (too high photon energy), complete decomposition of the organicacid and volatile metal product may occur. Typically, ultravioletincludes wavelengths below about 360 nm.

The source of ultraviolet light can be a 254 nm mercury discharge lamp.The liquid sample is preferably de-aerated.

A low molecular weight (LMW) organic acid is herein defined as anorganic acid of molecular weight less than 100 Daltons.

During the irradiation process, volatile reduced, hydrogenated and/oralkylated element compounds are formed and released from the sample inthe flow of carrier gas or as a result of their inherent vapour pressureand low solubility in the solution. Currently, volatile species of As,Bi, Sb, Se, Sn, Pb, Cd, Te, Hg, Ni, Co, Cu, Fe, Ag, Au, Rh, Pd, Pt, Iand S have been generated in this manner and specifically monitored anddetected. It appears that this approach may encompass many elements,such as those in Groups IIIA, IVA, VA, VIA, VIIA and IIIB, IVB, VB, VIBand VIII of the Periodic Table. The inventors have not yet identified acomplete list of elements that suitable, but such elements can bedetermined by routine experimentation. For example, it is believed thatBr and Cl would work well.

According to another aspect of the invention there is provided anapparatus for preparing an analyte for atomic spectrometry detection,comprising a spray chamber; an aerosol injector for introducing theanalyte into the spray chamber as an aerosol; a source of low molecularweight organic acid or other suitable photoactivatable ligan donorspecies; an ultraviolet radiation source for irradiating the analyte inthe chamber in the presence of the low molecular weight organic acid orother suitable photoactivatable alkyl donor species to create a reduced,hydrogenated and/or alkylated and/or elemental vapor containing theanalyte; and an outlet port for supplying the vapor containing theanalyte to an atomic spectrometry detector.

The invention takes advantage of the process of photoalkylation by UVlight in the presence of added low molecular weight organic acids toefficiently prepare a gas phase volatile form of a trace element toenhance the transfer of this form of the element to a cell used for itssubsequent detection by atomic emission, absorption, fluorescence ormass spectrometry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of the UV Reactive Spray Chamber.

FIG. 2 shows the effect of ultraviolet field on response from ⁷⁸Se, ¹²⁷Iand ²⁰²Hg during steady-state introduction of a 5 ng/ml multielementsolution containing 5% propionic acid. Vertical bars indicate onset andtermination of UV discharge.

FIG. 3 shows the effect of ultraviolet field on response from ⁷⁸Se, ¹²⁷Iand ²⁰²Hg during steady-state introduction of a 5 ng/ml multielementsolution containing 5% acetic acid. Vertical bars indicate onset andtermination of UV discharge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates one embodiment of the present invention, which is amodified commercial cyclonic spray chamber 10, typically used forpneumatic liquid sample introduction. It has a small pen lamp lowpressure mercury discharge lamp 12 inserted along the central axis ofthe spray chamber in such a manner as to not impede the normal operationof the spray chamber, including sample introduction, and waste removalby a waste drain 20.

Normally, the sample is introduced via a nebulizer, which is mounted inport 14 to result in the creation of a fine aerosol mist, 1-5% of whichtravels to the outlet port 16. This is connected to the remainder of thedetection system forming part of the atomic spectrometry equipment (notshown).

The aerosol can be created in a number of ways, such as pneumaticnebulization, hydraulic high pressure, thermospray, electrospray,ultrasonic, concentric and cross-flow liquid introduction systems.

When used in the preferred manner, a suitable concentration of LMWorganic acid (in the range 0.01 to 10 M) added to the sample before itis pumped into the spray chamber via the nebulizer port 14, whereupon itis exposed to ultraviolet irradiation from the mercury source 12. Insuch circumstances, the rapid reduction, hydrogenation and/or alkylationof many elements in the solution sample occurs and their gas-liquidphase separation from the solution is facilitated by the formation ofthe aerosol as well as aided by the normal nebulizer gas flow. Theresult is an enhanced efficiency of transport of the analyte element (upto 100%) to the detection system.

The embodiment of the UV reactive spray chamber as illustrated in FIG. 1provides for one means of achieving the desired production of volatileelement species for enhanced sample introduction efficiency. Alternativeforms may include physical variations of the spray chamber to addressall current commercial versions, such as the Scott, cyclone and conicaland those based on desolvation systems as well as include all means ofpneumatic and self-aspirating sample introduction systems, includingthose designed for integrated approaches to sample introduction whichtake advantage of pneumatic nebulization and hydride generation. See, R.L. J. McLaughlin and I. D. Brindle, A new sample introduction system foratomic spectrometry combining vapour generation and nebulizationcapacities, J. Anal. At. Spectrom., 17, 1540-1548 (2002), the contentsof which are herein incorporated by reference.

The arrangement for the UV source is also highly variable and can be asillustrated in FIG. 1 or physically envelope partially or completely thechamber walls containing the nebulized aerosol.

As an example, the current water cooling jacket 18 on the cyclonic spraychamber illustrated in FIG. 1 could form the envelope of a low pressuremercury discharge source, made more efficient by application of areflecting mirror to the exterior surface. Similarly, the source of UVenergy can vary in intensity for optimum application and the operatingwavelength should be less than 400 nm, although the optimum is the 253.7nm Hg resonance line.

EXAMPLE

A 50 ml internal volume water jacketed Twister cyclonic spray chamber(Glass Expansion, Victoria, Australia) was used. The standard wasteremoval line was modified to accommodate the mounting of a 6 W UVCmercury pen lamp (Analamp, Claremont, Calif., model 81-1057-51λmax 253.7nm) having a 50 mm lighted length and 5 mm o.d.

This was achieved by removing the handle and mounting the lamp barrel inthe ground glass fitting of the waste line using epoxy resin, asillustrated in FIG. 1. In operation, the lamp thus extended along thevertical central axis of the spray chamber and did not impede the normalpneumatic operation of the device. The spray chamber was fitted with aConikal concentric glass nebulizer (Glass Expansion, model 70115) andfed with sample via a peristaltic pump at a nominal flow rate of 1ml/min.

The nebulizer/spray chamber was mounted on the end of the torch with asocket attachment and supported an ICP in an Optimass 8000 TOF-MSinstrument (GBC Scientific Equipment Pty. Ltd., Australia). Typicaloperating conditions for the ICP-TOF-MS instrument are summarized in S.N. Willie and R. E. Sturgeon, Spectrochim. Acta, Part B, 2001, 56,1701-1716, the contents of which are incorporated by reference.

Formic, acetic and propionic low molecular weight organic (LMW) acidswere obtained from Anachemia and BDH and used without purification.Reverse osmosis water was further purified by deionization in amixed-bed ion-exchange system (NanoPure, model D4744,(Barnstead/Thermoline, Dubuque, Iowa) and nitric and hydrochloric acidswere purified in-house from commercial stocks by sub-boilingdistillation. Five ng/ml multielement solutions containing Ag, As, Ba,Bi, Cd, Cu, Pb, Hg, I, Sb, In, Ni, Sn and Se were prepared in highpurity water containing either 1% (v/v) HNO₃ or nominally 1 and 5% (v/v)LMW acids.

The ICP-TOF-MS was first optimized for response by introducing anapproximately 1 ml/min 10 ng/ml solution of Ho in 0.5% (v/v) HNO₃.Steady-state response from a multielement solution containing HNO₃ andfrom each of the three solutions containing the LMW acids was measuredwith and without the mercury discharge lamp on. In each case, theaverage response from 3 replicate 5 s integration periods was used. Thetemporal characteristics of the signals were also monitored using 1 scontinuous integration readings.

Sensitivities for all elements in the presence of the LMW acids weresignificantly lower than achieved with a nitric acid solution(5-50-fold), in part because instrument performance was optimized usinga nitric acid solution and the changes in density, viscosity, wettingcharacteristics and decomposition products associated with the LMW acidsolutions created non-optimum aerosol characteristics. It is possiblethat the benefits accruing from the use of the UV field, describedbelow, could be enhanced if sample introduction had first been optimizedfor each solution.

FIGS. 2 and 3 illustrate the time dependence of the evolution of theenhanced signals for ⁷⁸Se, ¹²⁷I and ²⁰²Hg when the mercury lamp ispowered, exposing the introduced aerosol to UV photolysis. Pronouncedchanges in the intensities of the signals for many elements were noted;these are summarized in Table 1. The suite of elements listed is notmeant to be comprehensive.

Most notable are the enhanced signals for elements such as Se, Bi, I, Hgand Pb in all LMW acids and Sb and Sn in formic and acetic acids. Bariumwas monitored as it is assumed to be unaffected by any alkylationreactions and changes in its intensity in the presence of the UV fieldlikely reflect physical alterations in the measurement system. Evolutionof carbon oxides as well as hydrogen and perhaps hydrocarbons may occurduring photolytic oxidation of the LMW acids which will change theoptimum sampling depth of the plasma and give rise to fluctuations inthe baseline and sensitivity of the system. Thus, to some degree, theeffects noted for Ba may be used to infer other physical changes in thedetection system that occur over and above those associated with realenhancements in sample introduction efficiency for some elements. Thesame observation is evident with the introduction of analytes in 1%nitric acid. Table 1 shows that, with the exception of Hg, UV photolysisresults in a nearly uniform 25% suppression in response for allelements. It may thus be inferred that evolution of molecular gases,such as nitrogen oxides, and/or the presence of the heated lamp post inthe spray chamber, gives rise to an alteration in the aerosoldistribution or composition, inducing a change in plasmachemistry/optimum sampling depth.

Photo-oxidation is a radical mediated reaction and response to thepresence/absence of the UV field should be immediate. Alkylation of anumber of elements may lead to production of reduced metal or halide andhydrides, methyl and ethyl analogues of the analyte in formic, aceticand propionic acids, respectively. The relatively slow rise and fall ofthe signals for these elements in response to the lamp being turned onand off is likely a consequence of the wetting of the internal walls ofthe spray chamber and the release of the volatile analyte species fromthe liquid phase. This is consistent with the increasingly longer timerequired to achieve steady-state response for Se, for example. As theLMW acid is changed from formic to acetic to propionic the “rise time”of the signal increases from 9 to 14 to 18 s. Earlier studies have shownthat such radical reactions lead to alkyl substitution onto the metal,resulting in hydride, dimethyl- and diethyl-Se compounds which areexpected to have correspondingly decreasing vapor pressures. Thus, adelay time, characteristic of sample wash-in and wash-out for a spraychamber, is evident in these experiments in response to powering the UVlamp on and off.

Mass 220 Da was also monitored in each system to reveal any changes inthe background over time. The influence of the UV field was difficult todetect as the total counts acquired were relatively small at this mass.All effects were significantly smaller than noted for Ba.

Table 1 summarizes the relative enhancement factors attained in thevarious LMW acids in response to the presence of the UV field. Datahighlighted in bold face indicate those elements for which an enhancedsensitivity is accorded to the presence of the UV field, the magnitudeof the effect surpassing any signal changes noted for Ba and assigned toplasma effects accompanying photolysis reactions.

TABLE 1 Relative intensity enhancement factors in response to UVphotoalkylation. Low Molecular Weight Acid Concentration % formic %acetic ——% propionic Element 1 5 1 5 1 5 1 % nitric Cu 1.0 0.9 1.4 3.31.7 1.8 .66 Ag 1.8 1.2 7.6 6.4 2.5 2.6 .67 Cd 1.0 1.2 2.0 3.9 1.9 2.0.70 As 1.1 1.7 1.6 4.4 2.0 2.6 .71 Se 2.8 16 19 29 5.6 6.3 .78 Ba 1.01.1 1.5 3.6 1.8 1.7 .72 Sb 1.0 9.3 2.9 4.6 2.0 2.3 .75 Hg 18 17 5.1 1617 17 1 I 2.2 3.1 12 38 12 16 .84 Bi 0.9 4.2 43 18 3.3 9.7 .77 Pb 1.02.0 7.0 5.9 2.5 3.1 .78 Ni 1.1 1.7 1.6 2.9 1.6 1.7 .67 Sn 1.0 5.6 3.25.2 2.1 2.1 .69 In 1.0 0.9 1.5 3.9 1.9 1.9 .69 *based on the relativeintensity change in the signal in the presence/absence of the UV field.The table headings need to be re-aligned

The combination of UV irradiation with pneumatic sample introduction ofsolutions containing LMW organic acids offers a simple and convenientapproach by which the benefits of photoalkylation can be easilyrealized. The influence of the intensity of the UV field requires studyas only a low power lamp was used for these experiments. Redesign of thespray chamber to create a full annular discharge, creating theultraviolet light within the space currently used for the water jacketor use of a larger surface area Scott-type spray chamber may enhanceefficiencies and minimize the “wash-in and wash-out” effects.

1. A method preparing an analyte for atomic spectrometry detectioncomprising: introducing an aerosol of the analyte into a chamber;irradiating the aerosol with ultraviolet light in the presence of a lowmolecular weight organic acid or other suitable photoactivatable liganddonor species to create a reduced, hydrogenated and/or alkylated and/orelemental vapor containing the analyte; and extracting the vapor fromthe chamber for use in atomic spectrometry.
 2. A method as claimed inclaim 1, wherein the chamber is a spray chamber.
 3. A method as claimedin claim 1, wherein the low molecular weight organic acid or otherligand donor species provides a concentration of 0.001 to 10 M.
 4. Amethod as claimed in claim 1, wherein the aerosol is irradiated in thepresence of a low molecular weight acid having a molecular weight <100Da.
 5. A method as claimed in claim 3, wherein the low molecular weightorganic acid is formic acid, acetic acid, or propionic acid.
 6. A methodas claimed in claim 1, wherein the low molecular weight organic acid orother suitable photoactivatable ligand donor species is added to theanalyte prior to formation of the aerosol.
 7. A method as claimed inclaim 6, wherein the aerosol is created with a nebulizer, and theanalyte is supplied to the nebulizer mixed with said low molecularweight organic acid or other suitable photoactivatable ligand donorspecies.
 8. A method as claimed in claim 1, wherein the other suitablephotoactivatable ligand donor species comprises a suitablephotoactivatable alkyl donor species.
 9. A method as claimed in claim 1,wherein ultraviolet light is created by an annular discharge surroundingthe chamber.
 10. A method as claimed in claim 9, further comprising areflecting surface to concentrate the light from said annular dischargeinto the chamber.
 11. A method as claimed in claim 1, wherein thewavelength of the ultraviolet light is 253.7 nm.
 12. A method as claimedin claim 1, wherein the analyte is an element selected from the groupconsisting of: Se, Bi, I, Hg and Pb and the ligand donor species is anLMW acid.
 13. A method as claimed in claim 1, wherein the analyte is anelement selected from the group consisting of: Sb and Sn and the liganddonor species is selected from the group consisting of: formic andacetic acids.
 14. A method as claimed in claim 1, wherein the analyte isan element selected from the group consisting of: As, Bi, Sb, Se, Sn,Pb, Cd, Te, Hg, Ni, Co, Cu, Fe, Ag, Au, Rh, Pd, Pt, I and S.
 15. Amethod as claimed in claim 1, wherein the analyte is selected from thegroup consisting of: metallic, metalloid, and halide elements.
 16. Amethod as claimed in claim 1, wherein the analyte is selected from thegroup consisting of groups IIIA, IVA, VA, VIA, VIIA and IB, IIB, IIIB,IVB, VB, VIB and VIII of the Periodic Table.
 17. An apparatus forpreparing an analyte for atomic spectrometry detection, comprising: aspray chamber; an aerosol injector for introducing the analyte into thespray chamber as an aerosol; a source of low molecular weight organicacid or other suitable photoactivatable ligand donor species; anultraviolet radiation source for irradiating the analyte in the chamberin the presence of the low molecular weight organic acid or othersuitable photoactivatable ligand donor species to create a reduced,hydrogenated and/or alkylated and/or elemental vapor containing theanalyte; and an outlet port for supplying the vapor containing theanalyte to an atomic spectrometry detector.
 18. An apparatus as claimedin claim 17, wherein said ultraviolet source is a mercury dischargelamp.
 19. An apparatus as claimed in claim 17, wherein said ultravioletsource is an annular discharge chamber around said spray chamber.
 20. Anapparatus as claimed in claim 19, wherein said ultraviolet sourceincludes a mirror reflector to concentrate ultraviolet light in saidspray chamber.
 21. An apparatus as claimed in claim 17, wherein saidaerosol injector is a nebulizer provided in an inlet port for the spraychamber.
 22. An apparatus as claimed in claim 17, wherein the aerosolinjector is connected to a supply of the analyte mixed with the lowmolecular weight organic acid or other suitable photoactivatable liganddonor species to provide said source.
 23. An apparatus as claimed inclaim 17, wherein said outlet port is connected to atomic spectrometrydetection equipment.
 24. An apparatus as claimed in claim 17, whereinsaid source supplies a low molecular weight acid.
 25. An apparatus asclaimed in claim 24, wherein said source supplies formic acid, aceticacid, or propionic acid.